This invention relates to a sono-chemical process that significantly improves the surface characteristics of beryllium enabling it to withstand several hundred hours of salt spray without corroding. Previous efforts to protect beryllium from corrosion have included costly anodizing and various, only partially successful attempts to apply passivation techniques. For example, U.S. Pat. No. 3,301,718 discloses a method for passivating beryllium using an aqueous solution containing potassium dichromate (K.sub.2 Cr.sub.2 O.sub.7) or chromic anhydride (CrO.sub.3) in combination with phosphoric acid. Both compounds contain hexavalent chromium ions, Cr.sup.+6, which is well known as a component of aqueous passivating solutions for a variety of metal surfaces. U.S. Pat. No. 3,827,919 describes a two-step process for cleaning and passivating beryllium surfaces. In the first step, corrosion is removed from the surface using an oxalic acid solution containing a surfactant. The cleaning step is followed by a passivating step in which the surface is treated with a solution containing phosphoric acid, hexavalent chromium ions, and a surfactant. The patent suggests using an ultrasonic treatment in the first cleaning step to insure thorough removal of corrosion products on the surface prior to passivating. U.S. Pat. No. 3,404,044 teaches a method for passivating zinc-containing surfaces using an aqueous acid solution containing hexavalent chromium ions, fluoride ions and an activator compound.
The above patents have been cited since they relate in some respects to the principles employed in the present invention wherein an aqueous solution contains an electron acceptor (CrO.sub.3) and a reagent (NaF) which behaves as a specific ligand for beryllium oxide. However, although ultrasonic sound has been employed (U.S. Pat. No. 3,827,919) to remove corrosion prior to passivation of beryllium, no prior process has employed the use of ultrasound (in a specifically controlled manner) in combination with a treatment bath, such combination producing a highly corrosion-resistant surface. We refer additionally to the special research report entitled "Electrodeposition of Metals in Ultrasonic Fields" by Kochergin and Vyaseleva, published by Vysshaya Shkola Press in Moscow (1964) and republished in 1966 by Consultants Bureau Enterprises, Inc., New York. This reference report deals with various phenomena induced by ultrasonic vibration in connection with the deposition of materials and electrolytic plating and is a definitive report in this field. However, the principle and method of the present invention (Dalton Process) has not been suggested or disclosed in the prior art.